High groundwood content newsprint furnishes have unique properties which make them difficult to deink with traditional caustic/surfactant chemistry. Newsprint inks consist primarily of carbon black in a mineral oil and resin vehicle which soaks into the cellulose fibers. Many groundwood based newsprint deinking processes require a combination of chemical and mechanical treatments which can be costly, time consuming and inefficient. They often have the tendency to drive a significant portion of the carbon black particles further into the fibers, which results in a gray pulp unsuitable for use in anything except low grade packaging paper. Furthermore, the use of highly alkaline pulping conditions cause significant yellowing of pulp.
Deinking of groundwood newsprint is of primary interest to mills producing book, magazine and newsprint stock. Many states are beginning to enact legislation requiring publishers to include up to 50% recycled newsprint in their newspapers or face tax penalties. Mills producing gypsum board and packaging are also interested in recycled news as a substitute for more expensive ledger grades and blank newsprint. The expense and long wait for the purchase of mechanical deinking equipment is increasing mill demand for chemical products, which will improve the deinking of a groundwood furnish with little mechanical treatment beyond the pulper.
Increased ink removal from newsprint during deinking is a function of the degree of wetting and dispersion of the carbon black ink particles. These particles are extremely hydrophobic and difficult to wet. Wetting is defined as the process that occurs when solid surface phase (paper and ink) and liquid phase come into contact, the solid-air interface being replaced by a solid-liquid interface. Wetting of the ink particles can be improved by lowering the surface tension of the water and decreasing the contact angle between the water and the particles. This leads to more particles being detached from the fibers and available for removal with the water. The wetting of carbon black particles can be increased, and deinking improved by choosing the proper surfactant. The use of surfactants in the deinking of newsprint has been disclosed in U.S. Pat. No. 3,377,234 and U.S. Pat. No. 4,311,352.
After the carbon black particles are detached from the fibers, the mechanical action of pulping and electrostatic interaction between the particles and paper fibers often causes the ink particles to become reattached in the many small crevices in and around the paper fibrils. This phenomenon may be minimized through the use of dispersants, which keep the ink particles suspended in the water. Dispersants function through two mechanisms: steric hindrance and charge interaction. By surrounding the ink particles with large molecules and increasing their negative charges, the proper dispersant can decrease the amount of ink redeposition on the fibers. The use of surfactants in combination with dispersants has been disclosed in U.S. Pat. No. 4,599,190. However, the surfactant/dispersant blend disclosed in this patent is strictly nonionic.